Douglas Isbell
Headquarters, Washington, DC March 13, 1998
(Phone: 202/358-1547)
Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)
Cynthia M. O'Carroll
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-6943)
RELEASE: 98-45
NEW GLOBAL SURVEYOR DATA REVEALS DEEPLY LAYERED TERRAIN,
MAGNETIC FEATURES AND GENESIS OF A MARTIAN DUST STORM
For the first time in Mars exploration, a spacecraft has
captured the full evolution of a Martian dust storm. NASA's
Mars Global Surveyor mission also has returned new insights
into the deeply layered terrain and mineral composition of the
Martian surface, and to highly magnetized crustal features that
provide important clues about the planet's interior.
These findings are among the early results from the Mars-
orbiting mission being reported in today's issue of Science
magazine.
This first set of formal results comes from data obtained
in October and November 1997, while the spacecraft was just
beginning to use the drag of Mars' upper atmosphere to lower
and circularize its highly elliptical orbit in a process called
aerobraking. At the time, a dust storm was brewing on Mars and
had grown to about the size of the South Atlantic Ocean.
The Global Surveyor data suggest that the event began as a
set of small dust storms along the edge of the planet's
southern polar cap, according to Dr. Arden Albee of the
California Institute of Technology, Pasadena, CA, the Mars
Global Surveyor mission scientist. By Thanksgiving, it had
expanded into a large regional dust storm in Noachis Terra that
covered almost 180 degrees longitude, while spanning 20 degrees
south latitude to nearly the tip of the Martian equator.
"As this storm obscured the Martian landscape, we followed
it in detail using several instruments onboard Mars Global
Surveyor," Albee said. "The Thermal Emission Spectrometer
mapped the temperature and opacity of the atmosphere while the
camera followed the visual effects. The effects of the storm
extended to great heights of about 80 miles (130 kilometers)
and resulted in great increases in both atmospheric density and
variability from orbit to orbit. These atmospheric measurements
have great significance to future Mars missions that will be
using aerobraking techniques too."
Before the storm, atmospheric dust was generally
distributed very uniformly, Albee said. Observations of the
limb of the planet in the northern hemisphere revealed both
low-lying dust hazes and detached water-ice clouds at altitudes
of up to 34 miles (55 kilometers). Movement of these clouds was
tracked by the spectrometer as the planet rotated. Atmospheric
turbulence disrupted these cloud patterns as the small storms
began to rise and kick more dust into the air. As the storm
began to abate, small local storms began to crop up again along
the edges of the south polar cap, and ice clouds formed in
depressions as the carbon dioxide cap continued to retreat.
In addition to these unprecedented observations of a full-
blown Martian dust storm, measurements from the spacecraft's
Magnetometer and Electron Reflectometer have yielded new
findings about Mars' strong, localized magnetic fields. These
patches of the crust, which register high levels of magnetism,
are beginning to unlock some of the mysteries surrounding Mars'
internal dynamo and when it died, said Dr. Mario Acuna of
NASA's Goddard Space Flight Center, Greenbelt, MD.
"These locally magnetized areas on Mars could not form
without the presence of an overall global magnetic field that
was perhaps as strong as Earth's is today," says Acuna. "Since
the internal dynamo that powered the global field is extinct,
these local magnetic fields act as fossils, preserving a record
of the geologic history and thermal evolution of Mars."
Magnetic fields are created by the movement of electrically
conducting fluids, and a planet can generate a global magnetic
field if its interior consists of molten metal hot enough to
undergo convective motion, similar to the churning motion seen
in boiling water.
"The small size and highly magnetic nature of these crustal
features, which measure on the order of 30 miles (50
kilometers), are found within the ancient cratered terrain
rather than within the younger volcanic terrain," Acuna said.
"By correlating crustal age with magnetization, we have a
perfect window on Mars' past, which will help us to determine
when Mars' internal dynamo ceased operating."
High-resolution images of dunes, sandsheets and drifts also
are helping reveal earlier chapters of Martian history.
Landforms shaped by erosion are almost everywhere, according to
Albee, and many bear a striking resemblance to
Colorado's Rocky Mountains. Rocky ridges poke through the
Martian dust just as the jagged edges of cliffs pierce through
a blanket of snow in the Rockies. Martian dust appears to have
spilled down the sides of ridges just as fresh snow slides down
a ski slope.
"One almost expects to see ski tracks crisscrossing the
area," Albee added. "These images present a sharp contrast to
the images of boulder-strewn deserts found at the Viking and
Pathfinder landing sites."
Newly released images from the Mars Global Surveyor camera,
developed by principal investigator Dr. Michael Malin of Malin
Space Science Systems, Inc., San Diego, can be viewed on the
Internet at:
http://www.jpl.nasa.gov/marsnews/img/march13.html
or
http://www.msss.com/mars/global_surveyor/camera/images/index.html
The Martian crust also exhibits much more layering at great
depth than was expected. The steep walls of canyons, valleys
and craters show the Martian crust to be stratified at scales
of a few tens of yards, which is an exciting discovery, Albee
noted. "At this point we simply do not know whether these
layers represent piles of volcanic flows or sedimentary rocks
that might have formed in a standing body of water," he said.
The Thermal Emission Spectrometer, led by principal
investigator Dr. Philip Christensen of Arizona State
University, is beginning to obtain a few infrared emission
spectra of the surface, although it is still too cold on the
surface for the best results. The best spectra clearly
indicate the presence of pyroxene and plagioclase, minerals
which are common in volcanic rocks, with a variable amount of
dust component. No evidence was found for carbonate minerals,
clay minerals or quartz. If present in these rocks, their
abundance must be less than about ten percent.
Their absence indicates that carbonates are not widespread
over the surface of the planet, but they may still be found in
specific locations that either favored their initial deposition
or their subsequent preservation. This finding could have
important implications for identifying areas that may preserve
signs of ancient life on Mars, since carbonate minerals are
commonly formed in biological processes, Albee said.
Striking results also have been obtained from Global
Surveyor's laser altimeter over Mars' northern hemisphere,
which is exceptionally flat with slopes and surface roughness
increasing toward the equator, according to principal
investigator Dr. David Smith of Goddard. The initial data for
this region helps scientists interpret a variety of landforms,
including the northern polar cap, gigantic canyons, ridges,
craters of all sizes and shield volcanoes. Most surprising are
views of extraordinarily mundane regions -- as flat as the
Bonneville Salt Flats in Utah - that extend over vast northern
regions of the planet.
Mars Global Surveyor will complete the first phase of its
two-part aerobraking strategy at the end of March, at which
time the science instruments will be turned on again for most
of the next six months. Over this period, the spacecraft will
stay in an 11 1/2-hour orbit and collect an additional bounty
of data at a closest approach of about 106 miles (170
kilometers) above the surface, much closer than the spacecraft
will pass over the planet once it has reached its formal
mapping orbit in March 1999. This closer orbit will allow the
science teams to take more detailed measurements of the Martian
atmosphere and surface without magnetic interference from the
solar wind.
"When we decided to slow the pace of aerobraking to reduce
the force on the solar panel that was damaged after launch, we
knew we would get a bonus -- the ability to collect much more
science data closer to the planet than will be possible during
the prime mapping mission," said Glenn E. Cunningham, Mars
Global Surveyor project manager at NASA's Jet Propulsion
Laboratory, Pasadena, CA. "Additionally, the six-month period
between the end of March and early September will yield an
extraordinary opportunity as the lowest point of the orbit
migrates over the northern polar cap. All of this information
that is coming back now is really icing on the cake, a
spectacular precursor to the global mapping data expected to
start flowing next year."
Mars Global Surveyor is part of a sustained program of Mars
explorationknown as the Mars Surveyor Program. The mission is
managed by the Jet Propulsion Laboratory for NASA's Office of
Space Science, Washington, DC. JPL's industrial partner is
Lockheed Martin Astronautics, Denver, CO, which developed and
operates the spacecraft. JPL is a division of the California
Institute of Technology.